Now to the bottom cap...Remember how you were told to drill a 7/8" hole in both 4" PVC caps? The reason for the hole in the bottom cap is so you can install a 3/4" PVC connector that is a threaded male on one end and a female slip fitting on the other end...The trick to installing a threaded fitting in an unthreaded hole? A simple blow dryer! Heat up the 7/8" hole with a blow dryer until the PVC becomes pliable and twist in your threaded fitting. Run cold water over it and let it set for a few minutes to harden again. No need for a thread tapping tool!